Recovery of organic isocyanates



A r1l28, 1959 I A. BLOOM ET AL RECOVERY OF ORGANIC ISOCYANATES FiledOct. 51'. 1956 mama-62v: 23:23 ucc 33.3mm

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Albert Bloom Harlan B. Freyermufh James B. Norrnington INVENTORSATTORNEYS rolysis products before 2,884,359 RECOVERY. OF ORGANIC ISOCYANATES Application October 31, 1956, Serial No. 619,382 10 Claims.(Cl. 202-52) This invention relates to an improvement in processes forthe production of organic isocyanates and more particularly to a processfor'recovering organic isocyanates from crude mixtures containing thesame.

It has long been known that organic isocyanates may be prepared byreacting phosgene with a primary amine corresponding to the desiredisocyanate. usually employed in the form of its hydrochloride, althoughthe free amine has been used in some cases. In place of phosgene,materials which liberate phosgene during the reaction may also beemployed. It is also known that potassium cyanate may be reacted with anorganic sulfate to produce the corresponding organic isocyanate. Anotherproposed method for producing organic isocyanates involves pyrolyzing anN-substituted carbamate and separating the corresponding isocyanate fromthe pythe latter have had time to reunite and form the startingN-substituted carbarnate. The foregoing reactions may be conducted inthe absence of an inert organic liquid diluent or in the presence of any.one of a large number of previously disclosed inert or- .ganic liquiddiluents.

During these aboveedescribed reactions for the production of organicisocyanates, which are often carried out at elevated temperatures, andduring subsequent steps for the treatment of the reaction mass torecover the organic isocyanates therefrom, which usually involves one ormore distillation steps, various by-products and polyrners'of thedesired organic isocyanates are produced which results in lower yield ofthe desired product. Although many attempts have been made to increasethe undesired by-products and residues remaining, no completelysatisfactory solution to the problem has thus far been found. Forexample, when a reaction mass is produced containing the desired organicisocyanate in an United States Patent F The amine is Y yield oforganicisocyanate and/ or reduce the amount of I inert organic liquid diluent,and the said diluent and the major portion of the isocyanate are removedby distillation, a substantial amount of by-product is obtained as aresult of polymerization and/ or other undesirable side reactions duringthe main reaction and the subsequent distillation, which by-product bothconsumes and entraps considerable quantities of the desired organicisocyanate. In US. Patent No. 2,680,128, a method is proposed for moreefficient recovery of organic diisocyanates by the addition of apolynuclear hydrocarbon plasticizer to the residue containing thediisocyanate followed by fractional distillation to recover thediisocyanate content. This plasticizer has been found to be oftendifiicult to recover (and reuse) and/or to use in such manner as toattain the desired increased yields.

It is an object of this invention to provide an improved method forrecovering an organic isocyanate from crude mixtures containing thesame. It is another object of to the production of an organicisocyanate. Other objects and advantages will appear as the-descriptionproceeds.

2,884,359 Patented Apr. 28, 1959 ice The attainment of the above objectsis made possible by the instant invention which comprises a process forrecovering the organic isocyanate from a crude mixture containing thesame comprising distilling said isocyanate from said mixture in thepresence of a saturated aliphatic hydrocarbon which is liquid under theconditions of the distillation. The process of this invention has beenfound to enable the attainment of increased yields of the desiredorganic isocyanate and to enable the recovery of organic isocyanatesfrom residues hitherto discarded. Although the mechanism by which theinstant process achieves the desired results is not definitelyunderstood, it is believed that the above defined liquid aliphatichydrocarbons employed herein may act (1) to increase the relativevolatility of the organic isocyanate whereby the isocyanate is morereadily volatilized from the distillation mass, (2) as a chemicalexerting a depolymerizing action, (3) as a heat exchange medium having adepolymerizing action, or (4) as a fluidizing agent for the byproducts,or in accordance with any combination of the foregoing mechanisms.

As examples of saturated aliphatic hydrocarbons which may be employed incarrying out the instant invention, there may be mentioned the normallyliquid or solid, saturated aliphatic straight or branched chainparaffinic hydrocarbons of at least 9 carbon atoms, or mixtures thereof,obtainable for example by various known synthetic methods and/or frompetroleum or from the distillation of the tar of wood, peat, lignite andbituminous shales, including substances commonly available and referredto as kerosenes, Vaselines, mineral oils, paraffin (e.g. lubricating)oils, mineral waxes, solid paraffins, and the like, and mixturesthereof. Specific compounds found in these hydrocarbons include nonane,octadecane, eicosane, pentatriacontane, and the like. In general, amongthe corresponding available isomers, those compounds having the normalstructure are preferred as having the higher boiling points. As stated,they must be liquid under the conditions of the distillation by whichthe isocyanate is removed from the mixture, which distillation may becarriedout under atmospheric but preferably under subatrnospheric orvacuum conditions and temperatures dependent upon the boiling point ofthe isocyanate under the same conditions. Stated otherwise, thesaturated aliphatic hydrocarbons employed herein must be stable, liquid,and have a boiling point higher than that of the organic isocyanatebeing recovered under the particular distillation conditions employed.The amount of saturated aliphatic hydrocarbon required to achieve thedesired results must be such as to yield a fluid distillation mass,which may be in the form of a solution, dispersion, or suspension or thelike. In most cases, however, an amount in excess of this minimum amountis employed since as pointed out above, the hydrocarbon may act inaccordance with any one or more of at least four different mechanisms.The optimum amount to be employed in any particular instance to achievethe desired maximum recovery of organic isocyanate will be readilydeterminable by the person skilled in the art and will depend upon theparticular organic isocyanate mixture being treated, the particularsaturated aliphatic hydrocarbon employed and the conditions ofdistillation, generally ranging from about 25 to 300%, and morepreferably about 50 to based on the weight of the organic isocyanate inthe mixture.

The process of the present invention has been found to be of particularinterest and particularly valuable in the recoveryoforganicdiisocyanates, particularly as present in a reaction massresulting from the reaction of phosgene with the corresponding aromaticdiamine or salt thereof in an inert organic liquid diluent. The instantprocess is particularly advantageous in cases wherein such inert organicdiluent has a boiling point below that of the organic isocyanatecontained therein under the conditions of distillation in accordancewith the instant process. However, it should be understood that theinstant process is equally applicable to the recovery of organicisocyanates generally; i.e., aliphatic, cycloaliphatic, alkyl aryl,aralkyl, heterocyclic and aryl mono-, diand polyisocyanates.Illustrative examples of these organic isocyanates which may berecovered in accordance with the process of the instant invention, byreaction of phosgene with the corresponding amine, are hexyl isocyanatefrom hexylamine, octyl isocyanate from octylamine, dodecyl isocyanatefrom dodecylamine, octadecyl isocyanate from octadecylamine,tetramethylene diisocyanate from tetramethylenediamine, pentamethylenediisocyanate from pentamethylenediamine, octamethylene diisocyanate fromoctamethylene diamine, undecamethylene diisocyanate fromundecamethylenediamine, dodecamethylene diisocyanate fromdodecamethylenediamine, 3,3'-diisocyanato dipropylether from3,3'-diaminodipropyl ether, etc., cyclohexyl isocyanate fromcyclohexylamine, tetrahydro-a-naphthyl isocyanate fromtetrahydro-a-naphthylamine, tetrahydro-fi-naphthyl isocyanate fromtetrahydro-flmaphthylamine, etc., Xylene diisocyanates fromxylylenediamines, diphenylmethane 4,4'-diisocyanate from4,4'-diamino-diphenylmethane, ,ipf-diphenylpropane 4,4'-diisocyanatefrom 4,4'-dian1inofl-diphenylpropane, etc., benzyl isocyanate frombenzylamine, phenylethyl isocyanate from phenylethylamine, p-isocyanatobenzyl isocyanate from p-amino-benzylamine, etc., phenyl isocyanate fromaniline, p-cetyl phenyl isocyanate from p-cetylaniline, p-dodecylphenylisocyanate from p-dodecylaniline, S-dodecyl-Z-methylphenyl isocyanatefrom S-dodecyl-o-toluidine, 3-nitro-4-dodecylphenyl-isocyanate from3-nitno-4-dodecyl aniline, pcetyloxyphenyl isocyanate fromp-cetyloxyaniline, metaphenylene diisocyanate from metaphenylenediamine, pphenylene diisocyanate from p-phenylenediamine, 1-methylphenylens-2,4-diisocyanate from 1-methyl phenylene-2,4-diamine,naphthylene 1,4-diisocyanate from 1,4- naphthylene diamine, 2,6-tolylenediisocyanate from 2,6- toluenediamine, 1,3,5-benzene triisocyanate from1,3,5- benzene triamine, etc., tetrahydrofurfuryl isocyanate fromtetrahydrofurfurylarnine, etc. It will be understood that the instantprocess is also applicable to the recovery of organic isocyanatesproduced by other reactions and from other components for example byprior art reactions discussed above.

In the preferred embodiment of this invention, the above definedsaturated aliphatic hydrocarbons is directly added to the crude liquidreaction mass resulting from the reaction of phosgene with thecorresponding aromatic amine or salt thereof in an inert organic liquiddiluent, and the resulting mixture subjected to distillation tosuccessively separate the said inert organic liquid diluent and theorganic isocyanate in relatively increased yields, the residueconsisting essentially of the defined hydrocarbon. An additionaladvantage of this preferred embodiment is that any by-products presentin the mixture or produced during the distillation remain in suspensionor solution in the presence of the hydrocarbon and are easily removedfrom the still in a fluid state. However, improved results are alsoobtained by addition of the hydrocarbon to the residue of organicisocyanate remaining after distilling ofi the inert organic liquiddiluent, or to the residue of by-products and organic isocyanateremaining after distilling ofi both the inert organic liquid diluent andthe major portion of organic isocyanate. It will be understood, however,that, the instant process is applicable for the recovery of organicisocyanates from other mixtures containing the same in liquid or solidform.

The distillation process of the instant invention is carried out in wellknown manner, and accordingly need not be described in great detail. Ingeneral, conditions of temperature and pressure (preferablysub-atmospheric or vacuum) employed in the process will be dependentupon the particular organic isocyanates being recovered, the othercomponents of the distillation mass, and the like.

The attached drawing is a flow sheet illustrative of various embodimentsof the process of the present invention. As shown, the desired resultsmay be obtained by adding suitable amounts of aliphatic hydrocarbon tothe system at any of the points indicated at arrows A, B or C. From theincoming crude liquid reaction mass containing organic isocyanate,by-products and inert organic liquid diluent there are obtained, asshown, separate fractions containing inert organic liquid diluent,organic isocyanate, and a residual mixture including byproducts andaliphatic hydrocarbon. It will be understood that any two successive, orall three, distillation steps shown may be carried out in well knownmanner in a single continuous distillation column, provided of coursethat the aliphatic hydrocarbon is added to the system prior to the finaldistillation step.

The following examples, in which parts are by weight unless otherwiseindicated are illustrative of the instant invention and are not to beregarded as limitative.

Example 1 Into a 2-liter, 4-neck flask is charged 609 grams ofo-dichlorobenzene. This solvent is cooled with stirring to 0-10 C. and140 grams of phosgene are passed in at this temperature. 122 grams of2,4-tolylene diamine are dissolved in 203 grams of o-dichlorobenzene byheating to C. This hot solution is then added dropwise to the phosgenesolution at 0-10" C. A fine slurry results. This slurry is stirred andheated to 100165 C. in about 2% hours While passing phosgene gastherethrough at a rate of approximately 2 grams per minute, at whichtime solution is complete. The phosgenation is continued for a shortadditional time, and the solution is then cooled and degassed with drynitrogen. The weight of the resulting crude solution of 2,4-tolylenediisocyanate in o-dichlorobenzene amounts to 960-980 grams.

Example 2 (For comparative purposes) 240 g. (one quarter) of the aboveresulting crude solution are placed in a 500 cc. round bottom fiask andthe o-dichlorobenzene distilled through a 15" glass column having adiameter and packed with glass helices. The distillation is carried outunder 1-2 mm. pressure. The o-dichlorobenzene is recovered at 35 to 40C., and the tolylene diisocyanate at to C. 33 g. of tolylenediisocyanate, analyzing 96% (73% of the theoretical yield) is obtainedwithout the use of a saturated aliphatic hydrocarbon.

Example 3 30 g. of a white paraflinic heavy mineral oil boiling at250-300 C. at 1-2 mm. pressure are added to 240 g. (one quarter) of theabove resulting crude solution of Example 1 and the o-dichlorobenzeneand tolylene diisocyanate distilled under vacuum as in Example 2. 36.8g. of tolylene diisocyanate analyzing 99.4% are obtained, whichcorresponds to 84.5% of the theoretical yield.

Example 4 240 g. (one quarter) of the. crude. solution obtained inExample 1 are distilled as in Example 2 to the point where theo-dichlorobenzene is recovered. 30 g. of the mineral oil employed inExample 3 are added to the still and the distillation to recover thetolylene diisocyanate carried out as in Example 2. A yield similar tothat of Example 3 is obtained.

Example The procedure of Example 2 is repeated until 31.3 g. of tolylenediisocyanate are obtained, at which time the 12 g. of residue remainingin the flask are still fiuid. 15 g. of the mineral oil employed inExample 3 are then added to the residue and distillation continued underthe same conditions to recover an additional ca. 6 g. of tolylenediisocyanate in a yield similar to that of Example 3.

In the above examples a mixture of 80% 2,4- and 20% 2,6-tolylenediamines may be employed as initial reactants instead of the 2,4-isomeralone, in which case a corresponding mixture of 2,4- and 2,6-tolylenediisocyanates is obtained.

This invention has been disclosed with respect to certain preferredembodiments, and there will become obvious to persons skilled in the artvarious modifications, equivalents or variations thereof which areintended to be included within the spirit and scope of this invention.

We claim:

1. A process for recovering an organic isocyanate from a crude mixturecontaining the same, comprising distilling said isocyanate from saidmixture in the presence of about 25 to 300%, based on the weight of saidisocyanate, of a saturated aliphatic hydrocarbon of at least 9 carbonatoms which is liquid under the conditions of the distillation.

2. A process as defined in claim 1 wherein said distillation is carriedout at sub-atmospheric pressure.

3. A process as defined in claim 1 wherein said mixture is in the formof a solution of the organic isocyanate in an inert organic liquiddiluent.

4. A process as defined in claim 1 wherein said organic isocyanate is anaromatic diisocyanate.

5. A process as defined in claim 1 wherein said organic isocyanate is2,4-tolylene diisocyanate.

6. A process as defined in claim 1 wherein said liquid saturatedaliphatic hydrocarbon is employed in proportions of about to of theweight of the organic isocyanate in said mixture,

7. A process as defined in claim 1 wherein said liquid saturatedaliphatic hydrocarbon is paraffinic mineral oil.

8. A process for recovering an aromatic diisocyanate from a reactionmass produced by reaction of phosgene with an aromatic diamine in aninert organic liquid diluent to produce the corresponding aromaticdiisocyanate comprising adding to said reaction mass about -25 to 300%,based on the weight of said diisocyanate, of a saturated aliphatichydrocarbon of at least 9 carbon atoms which is liquid under theconditions of the subsequent distillation, and then successivelydistilling said inert organic liquid diluent and said aromaticdiisocyanate from the reaction mass under vacuum.

9. A process as defined in claim 8 wherein said aromatic amine is2,4-tolylene diamine and said aromatic diisocyanate is 2,4-tolylenediisocyanate.

10. A process as defined in claim 9 wherein said liquid saturatedaliphatic hydrocarbon is paraffinic mineral oil.

References Cited in the file of this patent UNITED STATES PATENTS2,680,128 Slocombe et a1. June 1, 1954 2,680,129 Flores June 1, 19542,680,130 Flores June 1, 1954 2,706,733 Reid Apr. 19, 1955 2,757,183Irwin et al. July 31, 1956 2,819,949 Keller et al. Jan. 14, 1957 FOREIGNPATENTS 761,592 Great Britain Nov. 14, 1956

1. A PROCESS FOR RECOVERING AN ORGANIC ISOCYANATE FROM A CRUDE MIXTURECONTAINING THE SAME, COMPRISING DISTILLING SAID ISOCYANATE FROM SAIDMIXTURE IN THE PRESENCE OF ABOUT 25 TO 300%, BASED ON THE WEIGHT OF SAIDISOCYNATE, OF A SATURATED ALIPHATIC HYDROCARBON OF AT LEAST 9 CARBONATOMS WHICH IS LIQUID UNDER THE CONDITIONS OF THE DISTILLATION.